In the development of internal combustion engines, it is a basic aim to minimize fuel consumption, wherein improved overall efficiency is at the focus of the efforts being made.
Even though diesel engines, that is, one example of auto-ignition internal combustion engines, owing to the quality regulation which they use, have higher efficiency, that is, lower fuel consumption, than Otto-cycle engines in which the load is set by means of quantity regulation by means of the charging of the cylinders with fresh mixture, there is potential for improvement, and a demand for improvement, with regard to fuel consumption and efficiency in the case of diesel engines too.
One concept for reducing fuel consumption is deactivation of individual cylinders in certain load ranges. Efficiency of the diesel engine in part-load operation can be improved, by means of partial deactivation, because in case of constant engine power, deactivation of at least one cylinder of a multi-cylinder internal combustion engine increases load on other cylinders still in operation, such that said cylinders operate in regions of higher loads, in which specific fuel consumption is lower. The load collective in part-load operation of the diesel engine is shifted toward higher loads.
Cylinders which continue to be operated during partial deactivation furthermore tolerate higher exhaust-gas recirculation rates owing to greater mass of fuel supplied.
Further advantages with regard to efficiency result in that a deactivated cylinder, owing to the absence of combustion, does not generate any wall heat losses owing to heat transfer from combustion gases to combustion chamber walls.
Multi-cylinder internal combustion engines with partial deactivation described in prior art, and associated methods for operating said internal combustion engines, nevertheless have considerable potential for improvement.
The inventors herein have identified the above issues and potential for improvement in the operation of internal combustion engines. In one example, partial deactivation of auto-ignition internal combustion engine may be optimized, at least by an auto-ignition internal combustion engine comprising: at least two cylinders, in which at least two cylinders are configured so as to form at least two groups with in each case at least one cylinder, the at least one cylinder of at least one group being formed as a cylinder that can be switched in a load-dependent manner, wherein                at least two groups are characterized by different compression ratios εi,        at least one cylinder of a first group having a compression ratio ε1 and at least one cylinder of a second group having a compression ratio ε2, where ε2<ε1.        
Cylinders of at least one group are formed so as to be switchable, which permits not only an activation but rather, in particular, switching and if appropriate a later—then new—activation.
For example, during part-load operation of the internal combustion engine, that is, at low and if appropriate medium loads, at least one cylinder of one group is deactivated while at least one cylinder of at least one other group continues to be operated. If a predefinable load is undershot, a partial deactivation thus takes place, as a result of which load demand on at least one cylinder which remains in operation is increased, leading to advantages already described above. The power demand on the cylinders which are still in operation during the partial deactivation increases, such that said cylinders are operated at higher loads with a lower specific fuel consumption. Furthermore, a deactivated cylinder does not generate any wall heat losses, and thus does not contribute to efficiency losses as a result of heat transfer into the engine structure.
Additionally, since efficiency correlates with compression ratio of cylinder, part-load operation of the diesel engine may be further improved by having cylinder groups with different compression ratios.
In this way, by utilizing multiple cylinder groups with different efficiencies, part-load operation of auto-ignition internal combustion engine may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.